Twelve patients who presented with extranodal small lymphocytic proliferations were studied. The patients ranged in age from 33 to 67 years; six were male and six female. Locations of the initial lesion included the conjunctiva and orbit, lung, stomach, small bowel, skin, and subcutis. The histopathologic picture in all cases was that of a dense infiltrate of small lymphocytes with rounded nuclei; there were usually admixed plasma cells and sometimes germinal centers. Immunoperoxidase staining for immunoglobulin light chains demonstrated monoclonality in four cases and polyclonality in four; results were inconclusive or blocks were unavailable in the remainder. Follow-up ranged from 53 to 216 months with a median of 92 months. In three cases, there was no recurrence or recurrence only at the site of initial involvement; in three, there was a sequential involvement of different extranodal sites without significant morbidity; and in six, there was progressive disease. One of the latter patients developed nodular sclerosing Hodgkin's disease 48 months after diagnosis of the small lymphocytic process. There was no correlation between histopathologic findings and clinical source. All of the four patients with monoclonal immunoperoxidase staining and two of those with polyclonal staining had a progressive course; in the two with polyclonal staining, monoclonality was demonstrated later. It is concluded that the behavior of extranodal small lymphocytic proliferations cannot be reliably predicted by histopathologic criteria and that immunoperoxidase findings may be helpful as an indicator of an adverse course when monoclonality is demonstrated. Additional studies concerning this latter point are desirable.